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what does notch filter do?

You can also use it to boost a very specific frequency. But again, you have to know what you're after. Most general EQ's will boost a wide area IN the area that you are boosting. That's why it's Bass, Treble, and Mid. A notch filter allows you to finely focus the width of the surrounding area(that's called "Q") to give you the option of boosting or cutting a VERY specific area. A diagram of a general EQ would look like a rolling hill, and a notch filter would look more like a spike, or an extreme, and tall, ramp.

Fun fact: Wah pedals are basically MOVABLE notch filters. When you cut out the bottom frequencies, you have the wah pedal in the UP position. As you move the pedal down, the frequency that you're cutting moves into the higher range. Which in turn gives you the bass sound, or treble-cut, of a wah pedal in the down position.

Heck, if your bass has sweepable mids, turn the mid LEVEL to 10, and then run the FREQ knob back and forth. It's the exact same thing. Just a smaller area of frequencies. Makes for a nice greasy ending to a song.
 
Notch filters can be useful on your bass especially if you are using a bass that has a full spectrum sound. A Notch filter can help your bass mimic the sound characteristics of other basses. Alembic uses notch filters on most of their basses and some of the early EMG pickups also used a notch filter instead of a tone roll off. More recently Yamaha offered the NE-1 Parametric EQ that is essentially a notch filter. Some basses can benefit form a notch filter and others do not. I use a notch filter with my G-3, Dyna 5, a Carvin bass with old EMGs and my fretless bass but not on my ther basses.